1 - Field of the Invention
This invention relates to nuclear measurements involving the spectroscopic analysis of energy spectra of gamma rays resulting from the interaction of neutrons with atoms of elements constituting an unknown material. The invention can find application in nuclear well logging techniques, wherein a sonde is lowered in a well (or borehole) and carries out spectral measurements from which is derived information about the composition of the earth formation surrounding the borehole, or the borehole fluid, or the annulus including casing and cement located between the borehole wall and the formation.
2 - Related Art
A major goal of well logging is to obtain quantitative and qualitative information related to hydrocarbons in earth formation surrounding a well. A substantial part of nuclear well logging techniques are based on spectral analysis of energy spectra of gamma rays resulting from interactions of atoms with neutrons emitted from the sonde, such gamma rays being representative of certain atoms of the lithology (i.e. the matrix or the formation fluid) or of the borehole. Any reference hereafter made to "formation" or "lithology" has to be construed as referring to formation and/or formation fluid.
For example, the energy spectrum of gamma rays resulting from the capture of thermal neutrons, after being decomposed into contributions due to individual atomic elements, usually called "elemental yields", reveals information concerning the presence of earth formation elements such as e.g. hydrogen, silicon, calcium, chlorine, sulfur and iron. Important petrophysical parameters such as porosity, matrix lithology and water salinity may be derived from the elemental yields. Examples of capture gamma ray spectra analysis are depicted in U.S. Pat. Nos. 3,521,064 to Moran et al., 4,464,569 to Flaum, 4,507,554 to Hertzog and Nelligan, 4,661,701 to Grau, 4,810,876 to Wraight et al.; U.S. Pat. No. 4,937,446 to Roscoe, Stoller and McKeon shows an inelastic gamma ray spectral analysis. All the above mentioned patents are assigned to the assignee of the present application, and are as well incorporated herein by reference. In accordance with the teaching of the above identified Moran Patent, a measured gamma ray energy spectrum, representative of a formation of unknown composition, is compared with a composite spectrum constructed from individual laboratory derived standard spectra of the constituents postulated to comprise the formation. The different amounts of the standard spectra (elemental yields) which give the best fit to the measured spectrum when weighted by each element sensitivity (i.e. the ability of an element to emit gamma rays and be detected) represent the relative proportion of the constituents of the formation. By appropriate selection of the standards, the proportion of the constituents of interest can be obtained and the desired information regarding hydrocarbon content or lithology may be derived.
Capture gamma rays could also be used for determining the porosity of the formation, by using so called "neutron logs" which respond primarily to the amount of hydrogen in the formation. Thus, in clean formations whose pores are filled with water or oil, the neutron log reflects the amount of liquid-filled porosity. Neutrons are electrically neutral particles, each having a mass almost identical to the mass of a hydrogen atom. High-energy (fast) neutrons are continuously emitted from a radioactive source in the sonde. These neutrons collide with nuclei of the formation materials in what may be thought of as elastic "billiard-ball" collisions. With each collision, the neutron loses some of its energy. The amount of energy lost per collision depends on the relative mass of the nucleus with which the neutron collides. The greater energy loss occurs when the neutron strikes a nucleus of practically equal mass--i.e., a hydrogen nucleus. Collisions with heavy nuclei do not slow the neutron very much. Thus, the slowing of neutrons depends largely on the amount of hydrogen in the formation. Within a few microseconds the neutrons have been slowed by successive collisions to thermal velocities, corresponding to energies of around 0.025 eV. They then diffuse randomly, without losing more energy, until they are captured by the nuclei of atoms such as chlorine, hydrogen, or silicon. The capturing nucleus becomes intensely excited and emits a high-energy capture gamma ray. Depending on the type of neutron tool, either these capture gamma rays or the neutrons themselves are counted by a detector in the sonde. When the hydrogen concentration of the material surrounding the neutron source is large, most of the neutrons are slowed and captured within a short distance of the source. On the contrary, if the hydrogen concentration is small, the neutrons travel farther from the source before being captured. Accordingly, the counting rate at the detector increases for decreased hydrogen concentration, and vice versa. Examples of implementation of such method can be found in U.S. Pat. No. 4,816,674 to Ellis et al. or 4,423,323 to Ellis et al. both assigned to the assignee of the present application.
Atoms of the formation or the borehole could also be hit by neutrons in an interaction called "inelastic" wherein inelastic gamma rays are emitted. U.S. Pat. No. 4,507,554 to Hertzog and Nelligan, assigned to the assignee of this application, discloses a method of determining the composition of the borehole material in which an inelastic spectrum is recorded during the neutron burst and two capture spectra are obtained in respective time periods following the burst; one shortly after the burst and a second a much longer time after. The recorded spectra are analyzed as described above using sets of standard spectra specific to each time period. It is assumed that the earlier of the two capture spectra contains information about both the borehole and the formation, whereas the later capture spectrum contains information only, or at least primarily, about the formation. Accordingly, the difference between the constituent analyses derived from the capture spectra is taken to indicate the composition of the borehole. This technique has the disadvantage that the time period between successive neutron bursts may be relatively long, to allow the radiation emanating from the borehole constituents to subside sufficiently before the second capture spectrum is recorded. Consequently the logging speed must be relatively low, or alternatively poor depth resolution of the logs must be accepted. In addition, the assumption of little or no borehole contribution to the second capture spectrum is only an approximation, and thus does not necessarily reflect the real environment in which the spectral measurements are made.
It has been also proposed, as described in U.S. Pat. No. 4,788,424 assigned to the assignee of the present application, a method for producing an indication of the partition between a borehole and a formation of the constituents identified by means of a nuclear investigation. Capture gamma rays are detected and counted according to energy in each of two time gates. The resulting energy spectra are analyzed to determine the type and relative gamma ray yield of each constituent of the borehole and formation. A characteristic neutron capture decay time constant for each constituent is derived from the yields and total gamma ray counts in the two time gates, and time constants for the borehole and formation overall are set equal to the derived time constants for constituents, such as iron and silicon, occurring predominantly in the borehole and formation respectively. The partition of the remaining constituents is then determined by considering the characteristic time constant for each constituent to be the sum of the time constants for the borehole and formation regions weighted by the proportion of that constituent in each region, the borehole and formation time constants being assumed the same for all constituents and the sum of the proportions being unity.
Furthermore, examples of determination of lithology are depicted e.g. in U.S. Pat. No. 4,810,876 to Wraight et al., or in the U.S. patent application Ser. No. 476,223, filed on Feb 7, 1990, in the name of B. A. Roscoe and J. A. Grau, for a "Geochemical logging apparatus and method for determining concentrations of formation elements next to a borehole", both assigned to the assignee of the present application, or in the article entitled "Geochemical Logging with Spectrometry Tools" by R. Hertzog et al., presented at the 62nd Annual Technical Conference and Exhibition of the SPE, held in Dallas, Tex., on Sept. 27-30, 1987. Both the '876 patent and the SPE paper are herein incorporated by reference.
In order to penetrate the subterranean formation the fast neutrons must pass through the fluid contents of the borehole before entering the formation. The resulting borehole contributions to the inelastic and capture gamma ray spectra significantly complicate the analysis of the formation composition. One way of accounting for these contributions is to calibrate the logging tool in a reference borehole having known borehole contents and formation compositions. However, this requires large number of calibration measurements. Also laboratory conditions do not necessarily reflect the real composition of the contents of the borehole, so inaccuracies can result in the constituent proportions obtained from the spectra matching process. Taking more accurate account of the composition of an individual borehole's contents would enable more accurate information to be obtained concerning the constituents of the earth formations surrounding a borehole. Although the composition of the contents of the borehole may be determined with other logging tools, the use of the logs from such tools to correct the spectral analysis results requires accurate recording of the measurements and of the corresponding positions along the borehole. Separate borehole passes may be required for each measurement, contributing further to errors which arise from merging the data to assure depth correspondence. Each additional log requires additional expense and delay and contributes further errors.
A method for correcting for the borehole effect in inelastic gamma ray spectroscopy has been described in SPE paper "Response of the Carbon/Oxygen Measurements for an Inelastic Gamma Ray Spectroscopy Tool" by B. A. Roscoe & J. A. Grau, presented at the 1985 SPE Annual Technical Conference and Exhibit held in Las Vegas, Sept. 22-25, 1985. The depicted method aims at determining the parameters upon which depends the carbon-to-oxygen ratio and is based on the assumption that porosity and lithology are both known.
Although the above mentioned correction methods have proven to be useful, there is still a need for improvements towards a better knowledge of the borehole effects on the measurements the ultimate goal of which is to determine the characteristics of the formation, such as porosity, salinity or lithology.